Despite recent rapid progress in understanding the expression patterns and regulatory mechanisms of drug processing genes, namely drug metabolizing enzymes and transporters in adults, little is known about these in the pediatric period. The long-term goal is to understand mechanisms of ontogenic regulation of drug processing genes, so that efficacious and safe drug treatments can be achieved in children. Several factors are known to be essential for normal development, including hepatocyte nuclear factor 11 (HNF11), farnesoid X receptor (FXR), growth hormone (GH) signaling, and epigenetic influences. HNF11, a master regulator of early liver development, regulates hepatic expression of a large battery of drug processing genes. Initiation of bile-acid signaling pathways, mediated largely via the FXR, is a hallmark of perinatal liver development. GH is essential for postnatal hepatic gene expression and maturation. The accessibility of transcription factors to the target genes is largely determined by the methylation/acetylation status of histones and DNA sequences. Preliminary studies illustrate that in developing mouse livers, drug processing genes and transcription factors are expressed in distinct dynamic patterns and correlate with epigenetic signatures. The objective of this proposal is to elucidate the regulatory mechanisms of ontogenic expression of drug processing genes in mice. The rationale of this proposal is that its successful completion will generate basic knowledge that will serve as the foundation for further understanding pediatric pharmacology in humans. The central hypothesis is: developmental regulation of drug processing genes is a sequential event regulated by hormones, which activate transcription factors to modify epigenetic signatures and regulate gene expression. This hypothesis will be tested in 2 aims. Aim 1 will determine the ontogenic expression patterns of drug processing genes and the correlation with transcription factors and epigenetic signatures. The relative mRNA expression of major phase I/II enzymes and drug transporters in male mouse livers versus intestine and kidney will be examined, and correlated with expression of transcription factors and chromosome modifications (genome-wide DNA methylation and histone modifications). Aim 2 will elucidate roles of transcription factors and GH in determining ontogenic hepatic expression of drug processing genes in HNF11-null, FXR-null, and GH deficiency (lit/lit) mice using the same working strategy. This study is novel, because it will use a genome-wide approach to elucidate how alterations of hormones and transcription factors modulate epigenetic signatures and hepatic ontogenic expression of drug processing genes. This study is significant, because little is known about the regulation of hepatic drug processing genes in pediatric stages. Results from this study will: 1) provide basic knowledge on the ontogenic expression patterns of drug processing genes and nuclear receptors in liver, kidney, and intestine; and 2) help to understand how perinatal alterations in hormones and nuclear receptors, via modulating epigenetic signatures, affect stage-specific and long-term expression of drug processing genes.